Modern high performance ultrasound imaging systems, such as the SONOS.TM. 5500 that is manufactured by and commercially available from the Hewlett-Packard Company, U.S.A., are utilized for medical applications, among others. The latest designs of these systems use a wide bandwidth transducer to emit and receive ultrasound signals from an object (e.g., a patient) under test as well as a wide bandwidth digital beamformer and one or more filters for processing the data encoded on the ultrasound signals received from the object under test.
For ultrasound imaging systems implementing B-mode (grayscale brightness) imaging, such systems are typically configured to transmit a single pulse, approximating an impulse, and to use digital filters in the receive path to tailor the trade-offs of sensitivity, spatial resolution, and contrast (average gray level) resolution. In this regard, oftentimes, a digital bandpass radio frequency (RF) filter is implemented after the beamformer, and a digital low-pass post-detection filter is implemented downstream from the RF filter, typically after an amplitude detector.
There are many factors to consider in the design and operation of the RF and the post-detection filters. In general, a wider and smoother RF spectrum makes the speckle spatially smaller, because the coherence length is reduced; however, the RF filter bandwidth does not directly affect the speckle variance. A wider RF bandwidth can improve spatial resolution. The spatially smaller speckle can be averaged with the post-detection filter to improve contrast resolution.
Since the transducer aperture is typically only 32 to 64 wavelengths at the center frequency (and effectively even smaller at longer wavelengths), increasing the RF bandwidth by including lower frequencies can significantly degrade the lateral spatial resolution, because the lower frequencies are more weakly focused. The transducer filters both the transmit and receive signals, and the body attenuation of the signal is approximately proportional to the frequency (dB/MHz/cm) of the signal. This means that higher frequencies (which make a sharper image) usually have a much poorer signal-to-noise (S/N) ratio, so therefore, increasing the RF bandwidth by including higher frequencies can significantly degrade the sensitivity. The frequency dependent attenuation makes it desirable to have filters which dynamically change with depth as the echo is being received, either to improve performance as much as possible at each depth, or to create a uniform speckle texture. Furthermore, the generation of second harmonic signals in the propagation of the transmit pulse offers many opportunities for improved imaging, but brings along a whole new set of filtering trade-offs. Finally, with digital filters, aliasing artifacts must be considered.
In general, as is clear from the foregoing, the receive filtering is far too complicated to expect a system operator to understand or optimize it. Hewlett-Packard's SONOS 5500 ultrasound imaging system dealt with this issue by providing the operator with a simplified choice of 5 predetermined filter settings, or recipes. Each recipe on each transducer type was painstakingly developed by the system designers by empirically adjusting more than a dozen parameters while imaging many patients. While the SONOS 5500 ultrasound imaging system offers much better images and more versatility than previous systems, it is not optimum for any particular patient, and the time and effort to develop recipes for new transducers is daunting.
Thus, there is a need in the industry for a better way to implement filtering in an ultrasound imaging system.